Vulcanization accelerators are substances which shorten the vulcanization time or enable vulcanization to be carried out at a lower temperature. See, Ullmanns Encyclopadie der Technischen Chemie, 3rd Edition, Urban & Schwarzenberg, Munich-Berlin 1957, pages 383 et seq. Additional accelerators are sometimes used to develop full activity. Organic or inorganic activators can also be added. The inorganic activators commonly used are metal oxides, of which zinc oxide is particularly suitable.
So-called "EV-Systems" ("EV"=efficient vulcanization) are known for sulphur vulcanization and the vulcanization accelerators used in these systems are in most cases thiuramic compounds such as tetraalkyl thiuramic monosulphides (e.g. tetramethyl thiuramic monosulphide), tetraalkyl thiuramic tetrasulphides (e.g. tetramethyl thiuramic tetrasulphide) and, preferably, tetraalkyl thiuramic disulphides (e.g. tetramethyl thiuramic disulphide, referred to herein as "TMTD"). These thiuramic compounds have also been referred to as "ultra accelerators" on account of their excellent effect; in particular, they give rise to good hot air resistance in vulcanizates produced in their presence.
Thiuramic compounds have the property that either they, or the reaction products produced from them during vulcanization, tend to bleed, which produces undesireable deposits on the vulcanizate. The tendency to bleed depends, inter alia, on the nature of the rubber and on the concentration of the thiuramic compound. See, for example, W. Hofmann, in Gummi-Asbest-Kunststoffe 9 (1986), pages 422 et seq. The phenomenon of bleeding limits the amount by which the compression set can be increased by using larger quantities of thiuramic compound, particularly in rubbers having a low C.dbd.C double bond content.